MRI

Basic Principles and Applications

Brian M. Dale, Mark A. Brown, Richard C. Semelka

• Éditeur: Wiley-Blackwell
• Année: 2015
• ISBN:
  9781119013037
• Édition: 5
• Sujet: Medicina
• Sous-sujet: Radiología, radioterapia y medicina nuclear

This fifth edition of the most accessible introduction to MRI principles and applications from renowned teachers in the field provides an understandable yet comprehensive update.

• Accessible introductory guide from renowned teachers in the field
• Provides a concise yet thorough introduction for MRI focusing on fundamental physics, pulse sequences, and clinical applications without presenting advanced math
• Takes a practical approach, including up-to-date protocols, and supports technical concepts with thorough explanations and illustrations
• Highlights sections that are directly relevant to radiology board exams
• Presents new information on the latest scan techniques and applications including 3 Tesla whole body scanners, safety issues, and the nephrotoxic effects of gadolinium-based contrast media

Chapter 1
Production of net magnetization

Magnetic resonance (MR) is a measurement technique used to examine atoms and molecules. It is based upon the interaction between an applied magnetic field and a particle that possesses spin and charge. While electrons and other subatomic particles possess spin (or more precisely, spin angular momentum) and can be examined using MR techniques, this book focuses on nuclei and the use of MR techniques for their study, formally known as Nuclear Magnetic Resonance, or NMR. Nuclear spin, or more precisely nuclear spin angular momentum, is one of several intrinsic properties of an atom and its value depends on the precise atomic composition. Every element in the Periodic Table except argon and cerium has at least one naturally occurring isotope that possesses nuclear spin. Thus, in principle, nearly every element can be examined using MR, and the basic ideas of resonance absorption and relaxation are common for all of these elements. The precise details will vary from nucleus to nucleus and from system to system.

1.1 Magnetic fields

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Magnetic fields are produced by and surround electric currents, whether these currents are macroscopic currents such as those running through wires or microscopic currents such as those around an atom of iron. The magnetic field can be represented as a vector, meaning that it has both a magnitude and a direction, and is usually denoted by the variable B.¹ For example, the B field at the center of a circular loop of current-carrying wire points in the direction of the axis of the loop (perpendicular to the plane of the loop and therefore perpendicular to the current flow) and it has a magnitude that is proportional to the current in the loop. The magnitude of the field is related to the strength of the magnetic force on wires or magnetic materials, and the direction of the field is perpendicular to the direction of the force.

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Magnetic fields often vary over time and/or space, and will be coupled to the electric field, producing electromagnetic waves. Magnetic fields, particularly those in electromagnetic waves, are characterized by their frequency (the time between two consecutive “peaks” in the field). In MR, there are magnetic fields, which are constant in time, which vary at acoustic frequencies (a few kilohertz), and which vary at radio frequencies (RF) (several megahertz).

1.2 Nuclear spin

The structure of an atom is an essential component of the MR experiment. Atoms consist of three fundamental particles: protons, which possess a positive charge; neutrons, which have no charge; and electrons, which have a negative charge. The protons and neutrons are located in the nucleus or core of an atom; thus all nuclei are positively charged. The electrons are located in shells or orbitals surrounding the nucleus. The characteristic chemical reactions of elements depend upon the particular number of each of these particles. The properties most commonly used to categorize elements are the atomic number and the atomic weight. The atomic number is the number of protons in the nucleus and is the primary index used to differentiate atoms. All atoms of an element have the same atomic n...